Field of the Invention
This invention relates to processes for preparing ultra-high molecular weight polyethylene (“UHMW PE”) yarns, and the yarns and articles produced therefrom.
Description of the Related Art
Ballistic resistant articles fabricated from composites comprising high strength synthetic fibers are well known. Many types of high strength fibers are known, and each type of fiber has its own unique characteristics and properties. In this regard, one defining characteristic of a fiber is the ability of the fiber to bond with or adhere with surface coatings, such as resin coatings. For example, ultra-high molecular weight polyethylene fibers are naturally inert, while aramid fibers have a high-energy surface containing polar functional groups. Accordingly, resins generally exhibit a stronger affinity for aramid fibers compared to inert UHMW PE fibers. Nevertheless, it is also generally known that synthetic fibers are naturally prone to static build-up and thus typically require the application of a fiber surface finish in order to facilitate further processing into useful composites. Fiber finishes are employed to reduce static build-up, and in the case of untwisted and un-entangled fibers, to aid in maintaining fiber cohesiveness and preventing fiber tangling. Finishes also lubricate the surface of the fiber, protecting the fiber from the equipment and protecting the equipment from the fiber.
The art teaches many types of fiber surface finishes for use in various industries. See, for example, U.S. Pat. Nos. 5,275,625, 5,443,896, 5,478,648, 5,520,705, 5,674,615, 6,365,065, 6,426,142, 6,712,988, 6,770,231, 6,908,579 and 7,021,349, which teach spin finish compositions for spun fibers. However, typical fiber surface finishes are not universally desirable. One notable reason is because a fiber surface finish can interfere with the interfacial adhesion or bonding of polymeric binder materials on fiber surfaces, including aramid fiber surfaces. Strong adhesion of polymeric binder materials is important in the manufacture of ballistic resistant fabrics, especially non-woven composites such as non-woven SPECTRA SHIELD® composites produced by Honeywell International Inc. of Morristown, N.J. Insufficient adhesion of polymeric binder materials on the fiber surfaces may reduce fiber-fiber bond strength and fiber-binder bond strength and thereby cause united fibers to disengage from each other and/or cause the binder to delaminate from the fiber surfaces. A similar adherence problem is also recognized when attempting to apply protective polymeric compositions onto woven fabrics. This detrimentally affects the ballistic resistance properties (anti-ballistic performance) of such composites and can result in catastrophic product failure.
It is known from co-pending application Ser. Nos. 61/531,233; 61/531,255; 61/531,268; 61/531,302; 61/531,323; 61/566,295 and 61/566,320, each of which is incorporated by reference herein, that the bond strength of an applied material on a fiber is improved when it is bonded directly with the fiber surfaces rather than being applied on top of a fiber finish. Such direct application is enabled by at least partially removing the pre-existing fiber surface finish from the fibers prior to applying the material, such as a polymeric binder material, onto the fibers and prior to uniting the fibers as fiber layers or fabrics.
It is also known from the above co-pending applications that the fiber surfaces may be treated with various surface treatments, such as a plasma treatment or a corona treatment, to enhance the surface energy at the fiber surfaces and thereby enhance the ability of a material to bond to the fiber surface. The surface treatments are particularly effective when performed directly on exposed fiber surfaces rather than on top of a fiber finish. The combined finish removal and surface treatment reduces the tendency of the fibers to delaminate from each other and/or delaminate from fiber surface coatings when employed within a ballistic resistant composite. However, the effects of such surface treatments are known to have a shelf life. Over time, the added surface energy decays and the treated surface eventually returns to its original dyne level. This decay of the treatment is particularly significant when treated fibers are not immediately fabricated into composites, but rather are stored for future use. Therefore, there is a need in the art for a method of preserving the surface treatment and thereby increasing the shelf life of the treated fibers.